MOEPHOLOGY OF KEPEODUCTn'E ORGANS 185 



blue in some species of Ejiilobium, black in the Ttilip, rarely 

 green, and occasionally of a whitisbi tint. 



Besides the various markings just described as existing on 

 the extine, we find also either pores {fig. 387) or sUts {figs. 

 385, /, and 386, /), or both pores and slits, which vary in 

 number and arrangement in different plants. In the greater 

 number of Monocotyledons there is but one slit ; while three is 

 a common number in Dicotyledons. Sometimes there are six, 

 rarely four, still more rarelj- two, and in some cases we find 



Fig. 391. A. Pollen-grain of Picea exceUa, showing commencement of 

 germination, w, w. Tlie wings. 1. The large vegetative cell ; its proto- 

 plasm is contracted ; ■«', its nucleus ; 2, 3, the two collapsed cells ; 4 and 5, 

 the stalk cell and generative cell not yet completely divided from one 

 another, b. A little older grain, shomng germination a little further 

 advanced and the pollen-tube beginning to appear as an outgrowth of the 

 large cell, 1. Lettering as before, c. More advanced stage, showing only 

 upper part of the pollen-tube. The generative cell, 5, has divided into two, 

 each with a very large nucleus. x240. After Stiasburger. 



twelve or more slits. These slits are generally straight {fig. 

 385, /), but in Miintxlus moschatus they are curved. Other 

 still more complex arrangements occasionally occur. 



The pores, like the slits, vary in number. Thus we com- 

 monly find one in Monocotyledons, as in the Grasses ; and 

 three in Dicotyledons. Sometimes, again, the pores are very 

 numerous, in which case they are either irregularly distributed, 



